Posted
by
Soulskill
on Wednesday February 12, 2014 @03:44PM
from the now-you're-thinking-with-lasers dept.

sciencehabit writes "As it approaches its fifth birthday, the National Ignition Facility (NIF), a troubled laser fusion facility in California, has finally produced some results that fusion scientists can get enthusiastic about. In a series of experiments late last year (abstract 1, abstract 2), NIF researchers managed to produce energy yields 10 times greater than produced before and to demonstrate the phenomenon of self-heating that will be crucial if fusion is to reach its ultimate goal of 'ignition'—a self-sustaining burning reaction that produces more energy than it consumes."

The issue is that the previous management were attempting to achieve ignition through an engineering approach. They assumed that the science was well understood and all they needed to do was tweak the knobs and dials on the laser until they got the result they wanted.

When this spectacularly failed to work there was a change in leadership and the new guys are actually doing experiments rather than just firing the 'ignition' capsule over and over.

Then do as you say and you'll find out you're wrong. Example: Castle Bravo had a yield of 15 megatons, 10 of which were fission, only 5 were fusion. This is a common feature of actually weaponized "hydrogen" bombs and most of those devices tested by the US somewhere down in the Pacific where they didn't care what happened with the fallout.

The Soviets, on the other hand, realized that since they had to test on their own territory, they best reduce the fission yield of their test devices as much as possible to cut down the fallout. Their largest bomb hat a yield of 50 megatons, with only 1.5 megatons of fission yield. But they could have added 50 megatons of fission yield at any time by replacing the lead tamper with natural uranium (which was in fact the original plan) and presumably another 50 or so by using highly enriched uranium in its place.

D-T Fusion releases seriously fast neutrons (some 17 MeV) that can split any kind of uranium and makes fission much more efficient, because you don't need to rely soley on the chain reaction to give you enough neutrons before the whole thing blows apart.

Nobody really knows since nobody's ever got a significant amount of fusion to work for long enough to figure out the gas mileage, so to speak.

The golden chalice is deuterium-deuterium (D-D) fusion which can be done with just heavy water, D2O which is expensive but abundant (it makes up about 1 part in 40 million of water molecules), it just has to be separated out from regular water using centrifuges or other processes. Assuming a lot of fusion power plants are built then there would be be cost savings per tonne of deuterium produced the same way uranium mining and processing is a lot cheaper today than it was during WWII. D-D fusion is hard though.

The easier road to fusion is deuterium-tritium, so-called D-T fusion but tritium is only produced in small amounts in nuclear fission reactors. There is a way to produce tritium in a working fusion reactor by using a blanket of lithium to absorb neutrons but it's very experimental and unproven. ITER is being built in part to test the idea of Li breeding of tritium and it's likely JET will also be used to test the concept, it's being repurposed as a materials testbed for ITER.

The GP was correct. The vast majority of weapons that were actually built obtained most of their yield from fission. The exceptions were mainly tests and oddities like neutron bombs.

You don't get any fusion yield without surrounding it with something heavy to squeeze it, so the weight of the actual fusion fuel is irrelevant. They figured that if they needed something heavy in the bomb anyway, it might as well be uranium because that gave 2X to 3X the bang for free.

...D2O which is expensive but abundant (it makes up about 1 part in 40 million of water molecules)...

That 1 part in 40 million figure, while not wrong, is misleading.

Quoting Wikipedia:

It [duterium] has a natural abundance in Earth's oceans of about one atom in 6,420 of hydrogen.

So the hydrogen in the ocean is 1/6420 deuterium. Getting two of the deuteriums into one molecule is rare, but we don't really care. Chemical bonds are unimportant energetically compared to nuclear reactions.

"Not sure if this is a rehash of the same fusion discussion here a few weeks/months ago..."

In part.

The NIF did manage to spark a fusion reaction that actually output more energy than was input to the fuel pellet.

However, it is important to note that it was not more energy than the total input to the system. The energy used to power the lasers was still more than the energy of the fusion reaction. So it wasn't "break even".